1. Field of the Invention
The present invention relates to a golf ball and more particularly to an improvement in a dimple of the golf ball.
2. Description of the Related Art
A golf ball has approximately 200 to 550 dimples on a surface thereof. The role of the dimples resides in one aspect that such dimples disturb an air stream around the golf ball during the flight to accelerate the transition of a turbulent flow at a boundary layer, thereby causing a turbulent flow separation (which will be hereinafter referred to as a xe2x80x9cdimple effectxe2x80x9d). The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backward so that a drag coefficient (Cd) is reduced, resulting in an increase in the flight distance of the golf ball. In addition, the acceleration of the transition of the turbulent flow increases a differentia between upper and lower separating points of the golf ball which is caused by a back spin. Consequently, a lift acting on the golf ball is increased.
Examples of the specifications to greatly influence the flight performance of a golf ball include a total volume of dimples. The trajectory of a golf ball having a total volume which is too small tends to hop, and the trajectory of a golf ball having a total volume which is too large tends to drop. In any case, a sufficient flight distance cannot be obtained. In order to obtain a proper trajectory and a great flight distance, the total volume of the dimples is to be set within a predetermined range.
Various proposals for the density of a dimple have also been made. For example, Japanese Patent Publication No. Sho 58-50744 (U.S. Pat. No. 5,080,367) has disclosed a golf ball in which dimples are densely provided such that a pitch between the dimples is set to 1.62 mm or less if possible. Japanese Laid-Open Patent Publication No. Sho 62-192181 (U.S. Pat. No. 4,813,677) has disclosed a golf ball in which dimples are densely provided so as not to form a new dimple having an area which is equal to or larger than a mean area in a land portion other than the dimples. Japanese Laid-Open Patent Publication No. Hei 4-347177 (U.S. Pat. No. 5,292,132) has disclosed a golf ball in which dimples are provided very densely such that the number of land portions in which a rectangle having a predetermined dimension can be drawn is 40 or less. All the golf balls disclosed in the known publications have dimples provided densely, in other words, the surface area occupation ratio of the dimple is increased. The skilled in the art have recognized that the surface area occupation ratio is one of the important specifications to influence a dimple effect.
A golf player is very interested in making a good score and causing a golf ball to fly to a distance. A large number of golf players desire a golf ball which is excellent in a flight performance. As described above, various improvements have been made in relation to the total volume and surface area occupation ratio of the dimples. However, a golf ball to meet the demand of the golf player has not been obtained.
The present inventors have taken note of a mean curvature R as an important element to influence the dimple effect. The present inventors have found that an existing golf ball having a greater mean curvature R tends to be more excellent in a flight performance if a surface area occupation ratio Y is equal. By setting the relationship between the surface area occupation ratio Y and the mean curvature R to a range which cannot be obtained by the existing golf ball, the flight performance could be enhanced.
A golf ball according to the present invention has a large number of dimples on a surface thereof. A surface area occupation ratio Y of the dimples is 0.80 to 0.90. A mean curvature R of the dimples is 16 mm or more. The mean curvature R implies a mean value of a curvature r in the sectional shape of the dimple.
In a golf ball in which the surface area occupation ratio Y satisfies the range and the mean curvature R satisfies the range, the ratio of the occupation of dimples having comparatively large areas is high. The reason why the flight performance of the golf ball is excellent is not clear in detail. It is guessed that a dimple having a great curvature r contributes to a reduction in a drag coefficient (Cd), particularly, a reduction in the drag coefficient (Cd) in a high-speed region immediately after hitting.
It is preferable that a total volume V of the dimples should be 300 mm3 to 700 mm3. Such a golf ball presents a more excellent flight performance.
It is preferable that a mean occupation ratio y to be a value obtained by dividing the surface area occupation ratio Y by a total number N of the dimples should be 0.0022 or more. In such a golf ball, the ratio of the occupation of dimples having comparatively large areas is high. The golf ball presents a more excellent flight performance.
It is preferable that a sum X of a contour length x of the dimple and the surface area occupation ratio Y should satisfy a relationship indicated by an expression (I):
Xxe2x89xa63882xc2x7Y+1495xe2x80x83xe2x80x83(I).
Such a golf ball includes a dimple pattern having a smaller total contour length X for the surface area occupation ratio Y. The golf ball presents a more excellent flight performance.
It is preferable that a ratio of the number of dimples having a contour length x of 10.5 mm or more to a total number N of the dimples should be 91% or more. Such a golf ball presents a particularly excellent flight performance.